Cell polarization and focal accumulation of specific proteins to distinct membrane domains are important processes in the response of eukaryotes to attempted pathogen infection. These processes contribute to resistance to infection and are sometimes exploited by successful pathogens to gain entry into host cells. In humans, processes of cell polarization and focal protein accumulation play a role in the response to numerous pathogens including Pseudomonas aeruginosa, HIV, and the malarial parasite Plasmodium falciparum. Currently, the mechanisms underlying the recruitment of defense-related proteins to distinct locations on the cell surface in response to attempted pathogen infection are not well understood. In the model plant Arabidopsis thaliana, resistance to penetration and infection by the powdery mildew fungus Blumeria graminus f. sp. hordei (Bgh) is characterized by the focal accumulation of defense proteins at sites of attempted fungal penetration. Both PEN1, a syntaxin, and PENS, an ABC transporter, contribute to penetration resistance in Arabidopsis and are recruited to sites of interaction with the invading fungus. However, the mechanisms underlying the focal accumulation of these components of penetration resistance to sites of attempted fungal penetration are not known. Resistance of Arabidopsis to penetration by Bgh provides a novel model for studying the cell biological mechanisms underlying cell polarization and protein focal accumulation at sites of attempted pathogen infection. The specific aims of this research proposal are to: 1) Investigate the mechanisms underlying focal accumulation of the Arabidopsis PEN1 and PENS proteins at sites of attempted fungal penetration;and 2) Determine protein domains and posttranslational modifications important for the localization and defense function of the PENS ABC transporter. The proposed research will address the roles of roles of the cytoskeleton, vesicle trafficking, and lipid rafts in the focal accumulation of proteins at sites of attempted pathogen infection. Additionally, this research will address the role of elicitor-induced phosphorylation of an ABC transporter in the regulation of its activity and localization. Successful completion of these aims will improve our understanding of the mechanisms underlying cell polarization and protein focal accumulation during attempted pathogen infection.